Energy Efficient Extensions London 2026
Part L 2021 is tighter than most homeowners realise. Here is what you must meet, what you can do to go further, and how to turn your extension project into a whole-house energy upgrade.
Quick Answer
Part L 2021 requires wall U-values of 0.18 W/m²K and roof 0.15 W/m²K for new extensions. Complying adds £3k–£8k to build cost but cuts heating bills and improves EPC. The £7,500 Boiler Upgrade Scheme grant is available for heat pumps.
0.18 W/m²K
Wall U-value max
£7,500 BUS
Heat pump grant
£3k–£8k
Check your specific property constraints
Free Property CheckThe Regulatory Picture in 2026
Two sets of regulations govern the energy performance of London extensions in 2026: Part L (conservation of fuel and power) and Part O (overheating). Both apply to all extensions regardless of size.
The Part L 2021 update tightened the required U-values significantly compared with the 2013 version that was in force until June 2022. Many homeowners and even some builders are still quoting the old, lower standards. The table below shows what is currently required.
The Future Homes Standard (FHS), which would push performance further again, has been delayed. It was expected in 2025 but implementation is now likely 2026 or later for new dwellings, and even later for extensions. For now, Part L 2021 is what Building Control enforces.
The two regulations that apply to your extension
Part L 2021: Required U-Values for Extensions
U-value measures heat loss through a building element (watts per square metre per degree of temperature difference). Lower is better. These are the maximum permitted values under the current regulations for extensions to existing dwellings in England:
| Element | Max U-value (W/m²K) | Typical insulation solution | Old standard (pre-2022) |
|---|---|---|---|
| External walls | 0.18 | Full-fill cavity (100mm PIR) or 140mm timber frame with mineral wool | 0.28 |
| Flat roof | 0.15 | 150–170mm PIR above deck (warm roof construction) | 0.18 |
| Pitched roof | 0.15 | 200mm mineral wool between/below rafters (cold roof) or 120mm PIR between rafters (warm roof) | 0.18 |
| Ground floor | 0.18 | 100mm PIR under screed or 150mm EPS under slab | 0.22 |
| Windows and doors | 1.4 | Double-glazed argon-filled, aluminium or uPVC frame with thermal break | 1.6 |
| Rooflights / skylights | 1.7 | Double-glazed with low-e coating, thermally broken frame | 2.2 |
| Bi-fold / sliding doors | 1.4 | Thermally broken aluminium frame, double or triple glazed | 1.6 |
Common mistake: Many online cost guides still reference the old 0.26 or 0.28 W/m²K wall standard. If your builder is quoting based on these figures, their insulation specification is under the current requirement. Always ask for the U-value calculation document.
Going beyond the minimum: what a "well-insulated" extension looks like
Meeting minimum standards gets you compliance. Exceeding them costs relatively little at build stage (insulation is cheap compared with labour) but has a lasting impact on comfort and bills.
| Element | Minimum (Part L) | Well-insulated target | Extra cost approx. |
|---|---|---|---|
| Walls | 0.18 W/m²K | 0.12–0.14 W/m²K | £800–£2,000 |
| Roof | 0.15 W/m²K | 0.10–0.12 W/m²K | £600–£1,500 |
| Floor | 0.18 W/m²K | 0.10–0.13 W/m²K | £400–£1,000 |
| Windows | 1.4 W/m²K | 0.8–1.0 W/m²K (triple glazed) | £1,500–£4,000 |
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Part O: Overheating
Part O came into force in June 2022 and applies to all new extensions. It addresses the risk of overheating from solar gain, which is particularly relevant for single-storey extensions with large south or west-facing glazing, glass roofs, or roof lanterns.
There are two compliance routes. Your architect or designer will choose which one to use:
Route 1: Simplified method
A check based on glazing area as a proportion of floor area, plus the orientation of that glazing. If glazed area is less than 25% of the total floor area and faces predominantly north, the check often passes without further action.
Cost: No additional cost if your scheme passes. If it fails the simplified check, you move to Route 2 or redesign.
Route 2: Dynamic thermal modelling
A detailed computer model (CIBSE TM52 methodology) simulates the thermal performance of the extension through a year of weather data. Can demonstrate compliance for schemes that fail the simplified method.
Cost: £500–£1,200 for the modelling exercise. Often worth it to avoid redesigning large glazed schemes.
How to achieve Part O compliance
If your design has large south or west-facing glazing, these measures help demonstrate compliance:
- Solar-control glass: g-value below 0.35 significantly reduces solar gain. Adds £150–£400 per unit but often avoids the need for shading.
- Openable windows: Minimum free area for cross-ventilation. Check at design stage that your window specification allows adequate opening.
- External shading: Brise soleil, deep eaves, or external blinds. More effective than internal blinds at blocking heat before it enters the glass. Adds £1,500–£6,000.
- Reduce glazing area: Simplest fix. Replacing a full glass roof with a flat roof plus one or two structural skylights often resolves both Part O and thermal mass concerns.
Insulation in Practice: What Each Element Involves
Walls
Most London extensions use brick-and-block cavity construction with full-fill cavity insulation. To reach 0.18 W/m²K, the cavity typically needs to be at least 100mm wide, filled with either PIR (polyisocyanurate) rigid boards or full-fill mineral wool. PIR achieves better performance in a thinner profile but costs slightly more. Timber-frame extensions achieve the target more easily with 140mm studs packed with mineral wool plus a 50mm service void.
Flat roof
Warm roof construction (insulation above the structural deck) is the standard for flat-roof extensions and is required to reach 0.15 W/m²K. Cold roof construction (insulation between joists) cannot achieve this target without a very thick layer and is now unusual. A warm roof requires 150–170mm of PIR board above the deck, adding approximately £20–£30 per m² to roof cost compared with the old standard.
Ground floor
A concrete slab with 100mm PIR board beneath and a screed above is the typical solution. If specifying underfloor heating, the insulation goes below the heating pipes to direct heat upward rather than into the ground. Increasing insulation depth from the old standard to the new adds little cost at slab stage but becomes expensive to retrofit later, so always insulate properly now.
Glazing
The 1.4 W/m²K limit for windows requires double-glazed units with argon gas fill and a low-emissivity (low-e) coating, in a thermally broken frame. Most standard aluminium bi-fold and sliding door systems meet this without needing to specify triple glazing. Triple glazing (0.8–1.0 W/m²K) is available but adds 15–25% to glazing cost and significant weight to the door leaf.
Want to know if your extension design meets current energy standards?
Check your permitted development and energy requirementsHeating Options for London Extensions
An extension project is often the right time to review the whole-house heating system, especially if the existing boiler is more than 10 years old or has limited spare capacity. These are the main options:
| System | Install cost (inc VAT) | Grants available | Best suited to |
|---|---|---|---|
| Extend existing gas boiler system | £600–£1,800 | None | Boiler under 8 years old with spare capacity |
| Replace boiler + extend system | £2,500–£4,500 | None | Boiler near end of life; staying on gas |
| Air source heat pump (whole house) | £10,000–£18,000 | £7,500 BUS grant | Well-insulated home; UFH preferred; electric tariff |
| Electric underfloor heating (extension only) | £800–£2,000 | None | Small extension; occasional use; existing system inadequate |
| Wet underfloor heating (from boiler/ASHP) | £1,800–£4,000 | None (grant on ASHP) | Open-plan extension; best with UFH-compatible boiler |
| Infrared heating panels | £400–£1,000 | None | Very well-insulated extension; backup only |
Air source heat pumps and the £7,500 BUS grant
The Boiler Upgrade Scheme (BUS) provides a £7,500 grant toward the cost of an air source heat pump. The grant is applied directly by the installer and does not need to be claimed separately. To be eligible:
- The property must be in England, Scotland, or Wales
- The installer must be MCS-certified
- An EPC must exist for the property (Grade D or above for most applications)
- No outstanding loft or cavity wall insulation recommendations on the EPC (or evidence they are not suitable for your property)
Extension timing tip: An extension project is an ideal time to install a heat pump because the property is already disrupted and the UFH pipework can be laid in the new slab at minimal extra cost. Doing it later means lifting finished floors. Speak to an MCS-certified installer during the design stage.
Underfloor heating: wet vs electric
Wet UFH (water pipes from boiler or ASHP)
- Lower running cost than electric
- Works at low flow temp (ideal for ASHP)
- Higher install cost
- Slower response time
Cost: £1,800–£4,000 (extension only)
Electric UFH (mat or cable in screed)
- Lower install cost
- Fast response, zoned control
- Higher running cost on standard tariff
- Better with solar PV to offset cost
Cost: £800–£2,000 (extension only)
Your EPC and How the Extension Affects It
An Energy Performance Certificate (EPC) rates your home from G (worst) to A (best). Adding an extension affects your EPC score because the assessor models the whole home, not just the new part.
How an extension typically affects your EPC
Whole-house improvements worth doing while on site
With scaffolding up and builders on site, costs for the following are lower than when done in isolation:
| Improvement | Approx. cost (inc VAT) | Estimated annual saving | EPC impact |
|---|---|---|---|
| Loft insulation top-up (to 300mm) | £300–£600 | £150–£300 | +3–8 points |
| Cavity wall insulation (existing house) | £400–£900 | £200–£400 | +5–12 points |
| Replace existing single-glazed windows | £350–£700 per window | £50–£120 per window | +2–5 points per window |
| Solar PV (4kWp system) | £6,000–£9,000 | £600–£900 | +10–20 points |
| Smart heating controls | £200–£500 | £100–£200 | +2–4 points |
| Air source heat pump (inc £7,500 grant) | £3,000–£10,000 net of grant | £400–£900 vs gas | +15–30 points |
SAP Calculations: When You Need One
A SAP (Standard Assessment Procedure) energy calculation models the thermal performance of the whole house including the extension. It is required by Building Control in two situations:
- The extension adds more than 25% to the total floor area of the existing house (in which case the extension is treated similarly to a new build for energy purposes)
- The glazed area exceeds 25% of the new floor area and the designer is using the whole-dwelling carbon emissions calculation (SAP) to demonstrate Part L compliance rather than the element-by-element approach
For most single-storey extensions (rear, side return, garage conversion), the element-by-element approach applies and a full SAP is not required. Your architect or energy assessor will confirm which route applies. A SAP calculation costs £300–£600 and is usually carried out by a certified energy assessor.
Solar PV: Worth Adding During the Build?
Adding solar PV panels while scaffolding is already up costs significantly less than a standalone installation. The scaffold hire (£800–£2,000) is the major cost element for a standalone job; bundled with an extension, the marginal cost is just the panels and inverter.
Solar PV economics for London extensions 2026
Solar PV combined with a battery storage system (£3,000–£5,500 for a 5kWh battery) raises self-consumption from around 40–50% to 70–80%, improving payback further. If you are also installing a heat pump or electric vehicle charger, the case for PV + battery becomes compelling.
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5 Common Energy Efficiency Mistakes
1. Using the old 0.28 or 0.26 W/m²K wall spec
The Part L 2021 wall target is 0.18 W/m²K, not 0.26. Some online guides and even some builders are still quoting pre-2022 values. If your builder's specification doesn't reference a U-value calculation document, ask for one before work starts. Building Control will check.
2. Choosing a full glass roof without addressing Part O
A glass roof on a south-facing extension almost certainly fails the Part O simplified check. This is discovered late, after the design is agreed and the planning application submitted. Check Part O compliance at the design concept stage, before the planning drawings are finalised.
3. Specifying electric UFH without solar PV
Electric underfloor heating at standard tariff rates costs roughly three times more to run per kWh than a gas boiler and twice as much as a heat pump. It makes sense as a supplementary system or in a very well-insulated extension, but as a primary heat source it can add £300–£600 per year to energy bills compared with a wet system.
4. Missing the opportunity to insulate the existing house
With builders on site and scaffolding potentially in place, topping up loft insulation or installing cavity wall insulation costs a fraction of what it would as a standalone job. Many homeowners focus entirely on the new extension and miss this window. Ask your contractor or architect to include a whole-house improvement quote.
5. Not laying UFH pipework in the slab even if not connecting it immediately
If you're not installing a heat pump now but might in the next 5 years, laying the UFH pipework in the slab during the build costs roughly £500–£900. Doing it later means breaking up the finished floor at a cost of £3,000–£8,000. Plan for future systems even if you don't install them now.
Frequently Asked Questions
What are the current Part L U-value requirements for extensions in 2026?
Under Part L 2021 (current in England): walls 0.18 W/m²K, flat roof 0.15 W/m²K, pitched roof 0.15 W/m²K, floor 0.18 W/m²K, windows and doors 1.4 W/m²K, rooflights 1.7 W/m²K. These are tighter than the pre-2022 standards that many online sources still quote.
Do I need a SAP energy calculation for my extension?
Usually not for a standard single-storey extension. A SAP calculation is required when the extension adds more than 25% to the total floor area of the house, or when glazing exceeds 25% of floor area and you are using the whole-dwelling emissions approach to demonstrate Part L compliance. Your architect or Building Control officer will confirm which route applies.
What is Part O and does it apply to extensions?
Part O (overheating) applies to all new extensions, not just new-build homes. It addresses the risk of summer overheating from solar gain. Extensions with large south or west-facing glazing, glass roofs, or roof lanterns are most likely to be affected. Compliance can be demonstrated via a simplified check or dynamic thermal modelling.
Can I get the £7,500 heat pump grant if I'm having an extension?
Yes, if you meet the Boiler Upgrade Scheme eligibility criteria: the property must be in England, Scotland or Wales; the installer must be MCS-certified; an EPC must exist; and there must be no outstanding insulation recommendations on the EPC that can be addressed (or evidence they are unsuitable). The extension project itself does not affect eligibility.
Will my extension reduce my EPC score?
Adding floor area without improving the heating system tends to slightly reduce your EPC score because the assessor models the whole home. However, if you upgrade to a more efficient heating system or combine the extension with whole-house improvements (loft insulation, cavity wall insulation, solar PV), the net effect can be an improvement in your EPC rating.
Is underfloor heating worth it in an extension?
Wet UFH (from a boiler or heat pump) is very well-suited to open-plan extensions with large floor areas and is especially efficient when run at the low flow temperatures that heat pumps prefer. Electric UFH is lower cost to install but higher cost to run. Both are more comfortable than radiators in open-plan spaces because heat is distributed evenly across the floor.
How much extra does meeting Part L 2021 add to build cost?
Typically £3,000–£8,000 for a standard single-storey extension, compared with building to the old 2016 standard. Most of the additional cost is in thicker insulation (modest) and higher-specification glazing (most significant element). The cost is recovered over 5–10 years through lower energy bills.
Should I add solar PV during my extension build?
The extension build is an ideal time to add solar PV because scaffolding costs are shared. A 4kWp system added during an extension build costs £4,500–£7,000 compared with £6,500–£9,000 as a standalone installation. Payback in London is typically 6–10 years. If you plan to install a heat pump or EV charger, the case for solar becomes stronger.
Summary
Every London extension in 2026 must comply with Part L 2021 (walls 0.18 W/m²K, roof 0.15 W/m²K) and Part O overheating. These are non-negotiable and will be checked by Building Control. Getting them right adds £3k–£8k to build cost but reduces running costs for the life of the building.
The extension project is also the best opportunity to improve the whole-house energy performance. With builders on site, topping up insulation, replacing old windows, and installing solar PV or a heat pump all cost significantly less than standalone jobs.
For a full picture of extension costs including energy compliance, see our London extension costs 2026 guide. For the full building regulations framework, see our building regulations for extensions 2026 guide.
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